An impact test referred to as a bullet test is performed on various motor cases to ascertain the time lapse between impact of a bullet and explosion of the impacted motor case.
A bullet test involves shooting a bullet into a live piece of missile propellant encased in some type of material that simulates a motor case. The bullet strike may cause the motor case to detonate. The problem to be solved is measuring the absolute time lapse between the bullet impact and the motor explosion. A standard shock wave, upon emanating from the point of a bullet impact in missile propellant, is generally about 5 millimeters per microsecond. Therefore, the time lapse measurement must be at least in the microsecond range to be meaningful. This is an extremely accurate measurement and any inaccuracy in the data gathering devices will lead to a large, indeterminable error.
The main reason for timing errors that occur in conventional solutions is that most of the techniques involve the use of a shock sensing device to detect the impact of the bullet and then, the shock of the detonation. Methods like these do not give the required accuracy because of a fundamental flaw of the methods that is due to the speed of a vibration wave traveling through the motor casing. The sensing devices are usually placed several inches away from the expected point of impact for protection purposes. When the bullet strikes the motor case, a three dimensional shock wave emanates from the point of impact and travels through both the motor casing and the propellant itself. The wave travels at a finite velocity through the various materials and it takes a certain length of time for the shock waves to actually reach the impact sensing device where they can be recorded. It is very possible, given the fact that there is an appreciable time lag between the time when the bullet strikes the case and the time when the impact is recorded, that the propellant can react to the shock and detonate in less time than it takes for the initial shock wave of the bullet strike to reach the sensing devices. The data in such a situation would show that the motor detonated before the bullet impacted the case. In an experiment requiring high degrees of accuracy, it is immediately apparent that a simpler, much more accurate way of timing the span between bullet impact and motor detonation is required. The detonation properties of various propellants are revealed by such time lapse measurements. This knowledge may, in turn, be used to manufacture improved propellants.